1. Field of the Invention
The present invention relates to small, thin plate antenna, which can be incorporated in an electric appliance such as portable terminal, electric product or wall or the like, composed of a conductive plate and an electric appliance therewith.
2. Prior Art
Recently, reduction in size of various kinds of special purpose antennas for portable phone, mobile computer and the like (hereinafter referred to as portable terminal), has been progressively executed except a large size antenna for base station, satellite broadcasting and the like. Particularly, the antenna for portable terminal has increased problems in its installation space, that a higher performance is apt to be demanded against the restriction of its antenna volume and other things, accompanied by reduction of the terminal itself. Additionally, radio network concept at home which has been aggressively considered recently, contains the same problem in the size of the antenna with introduction of the antenna into indoor wall surface, personal computer, other electric products and the like (hereinafter, referred to as electric product).
If a dedicated antenna is incorporated in a case or main body case (hereinafter referred to as casing) of a portable terminal or electric product, the above-mentioned problem is generated because a new dedicated space must be secured. Further if reduction in size or weight of the product is accompanied, naturally reduction in volume and weight of the antenna itself is required and thus, a demanded antenna performance is difficult to satisfy. That is, the casing needs an appropriate installation space in order to incorporate the antenna in the casing and secure its performance. Consequently, each specification used up to now need to be changed, so that increase in manufacturing cost of the product orprolongation of development period occurs. For the reason, to avoid this problem, a separate casing is used outside the main body casing in most cases and an externally attached antenna, which is to be attached through an additional cable, is used. However, according to this method, if that portable terminal or electric product is moved, the externally attached antenna must be removed in most cases. Further, labor for reinstallation and readjustment is needed and depending on case, the cable must be placed around or a physical fault due to unexpected trouble may be generated and freedom in the installation position of such a portable terminal or electric product may be restricted, so that user always feels inconvenient with such a plate antenna.
In order to solve these problems, Japanese Patent Application Laid-Open No. HEI5-22018 and Japanese Patent Application Laid-Open No. HEI8-256009 have disclosed typical examples of thin incorporation type antennas which can be incorporated in a gap or the like in a casing of a portable terminal or electric product. These well-known antennas are thin and easy to manufacture. However, in order to obtain a high radiation gain with these well-known antennas, a wide grounding portion is needed, so that consequently, the structure is likely to be increased. To secure a high radiation gain and reduce the size, a grounding portion in the high frequency circuit portion within the casing or a grounding conductor is connected to a grounding portion of the antenna directly with metallic screw or welding in terms of high frequency, so that current distribution on the antenna is allowed to exist on this conductive portion. Finally, these ground portions in the casing need to be used as part of the ground portion of the antenna. That is, in the antenna of the above publication, the ground portion of the antenna needs to be connected to the ground portion in the casing directly through a metallic screw or welding at the antenna installation position or a space portion. Consequently, this type of the antenna is not suitable for demand for reduction in size or weight of the product and if achieved, that product is lack of general-purpose performance.
For the reason, each dedicated antenna, which is to be incorporated in the portable terminal or radio network household product, needs to be introduced easily without increase of manufacturing cost or prolongation of development period and reduce user's inconvenience upon use of that product. Further, the antenna itself needs to be of low cost.